Quantum key distribution (QKD) involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals (“photon signals”) transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the photon signals will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984), and in the article by Bennett et al., “Experimental Quantum Cryptography,” J. Cryptology, (1992) 5: 3-28, which articles are incorporated by reference herein. A specific QKD system is described in U.S. Pat. No. 5,307,410 to Bennett (the '410 patent), which patent is incorporated by reference herein.
The above-mentioned publications and the '410 patent each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of the photon signals, and Bob randomly measures the polarization or phase of the photon signals. The one-way system described in the '410 patent is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
When operating a commercial QKD system, multiple variables need to be aligned in time and then maintained aligned for optimum system performance. For example, in a commercial QKD system one or more single-photon detectors (SPDs) are gated with a gating signal from a controller to synchronize the detection of optical pulses with expected pulse arrival times. However, once the system is set up, the timing drifts due to various systemic and environmental factors. This causes the photon count to drop, which in turn leads to a reduction in the transmission rate of the system, and to an increase in the bit-error rate (BER)—i.e., to less-than-optimum system performance.
While laboratory and prototype QKD systems can be adjusted to account for system drifts under very controlled and artificial conditions, making the necessary adjustments to maintain optimum or near-optimum performance of a commercial QKD system in the field is a far more daunting endeavor. And, unlike with a laboratory or prototype QKD system, end-users of commercial QKD systems have an expectation that their QKD system will automatically run in an optimum state with minimal or no operator intervention.